Parents and grandparents will be allowed to draw on their pension pots to secure deposits for young family members to help them get a foot on the housing ladder, Nick Clegg has announced.

In an interview with The Andrew Marr Show on BBC1, the Liberal Democrat leader and deputy prime minister promised to help thousands of young people desperate to get their feet on the first rung of the property ladder but who are prevented by lack of deposits.

In a wide-ranging interview in Brighton to mark the party's annual conference, Clegg also:

• Dismissed pressure from his own party in the face of dismal poll ratings and ruled out standing down before the next general election. "I'm not going to flinch, I don't think you should flinch when you're on a journey," he said.

• Said he was confident he could get the Conservatives to sign up to a new wealth tax before 2015 but acknowledged that he had so far failed to convince his coalition partners to introduce a mansion tax.

• Revealed he had had "lengthy conversations" in recent weeks with Ed Miliband, David Miliband, Tony Blair and Peter Mandelson;

• Underlined the Lib Dems' determination to make things fairer as he made clear that his party would resist any move by the Conservatives to bring in further belt-tightening measures in a way that only hit the poor.

"I will not accept a new wave of fiscal retrenchment, of belt-tightening, without asking people at the top to make their contribution, to make an additional contribution," Clegg told Marr. "I don't think you can ask people on middle and low incomes, who, after all, are the vast majority of the British population, to bear the brunt of this adjustment."

On the new plan for people to be able to guarantee family members' mortgages using their pension pots, he said: "This is part and parcel of something which I think most people agree with, which is that as we fill in the black hole in the public finances we have also got to make sure that we do not put Humpty Dumpty back together again and make the same mistakes, that we rewire the British economy and make it fairer and give people more opportunities."

The Lib Dem leader said he would continue to try to persuade David Cameron and George Osborne to "do the right thing" by introducing a mansion tax on houses worth £2m or above, he said. "So far I have failed to do so. I will continue to make the argument, and there are a number of Conservative MPs who agree. The mansion tax is not the only way in which you can make people at the top make a fair contribution to this huge national effort of balancing our books, and we've already demonstrated to you, through capital gains tax, through stamp duty, through tax avoidance, already ensure that the top 10% pay more, and we can do more than that."

He added: "I think the vast majority of people in this country would find it wholly unacceptable if further fiscal austerity was basically implemented on the backs of the poor. Most people in this country are very fair-minded; they understand we're in the middle of a very difficult journey of repairing, rescuing, restoring our British economy, and they want us, and they want particularly Liberal Democrats in government, to fight for the fairest possible way of doing that. Let me be clear: I'm not saying that something as big as welfare, which is about a third of government expenditure, is immune from further savings, but I'm saying that the burden has to spread fairly."

The second day of the conference has been dogged by poor poll ratings, including an Observer survey which puts the Lib Dems fourth behind Ukip. A separate survey of the party's grassroots by Liberal Democrat Voice suggests Clegg has lost the confidence of party members, with his personal rating falling below zero to minus two, while confirming huge support for the business secretary, Vince Cable.

The Independent on Sunday showed less support for Cable in the wider public, with only 27% saying he would make a better leader, compared with 25% who disagreed. But 48% of all voters view Clegg as inexperienced and only 3% judge him to be good in a crisis.

Asked by Marr about reports that some of his colleagues had been texting senior Labour figures, he said: "Grown-up politicians talk to each other across party lines. Over the last few weeks I have had lengthy conversations with Ed Miliband, David Miliband, with Tony Blair, with Peter Mandelson … talking about Europe, talking about political reform."

The mortgage deposits scheme is being looked at by both the Treasury and the Department for Work and Pensions and would typically involve borrowing around 25% of the value of the lump sum. So a pension pot of £40,000 would allow someone to borrow a deposit of £10,000 for a child or grandchild.

Mortgage lenders and pension funds have been approached and seem favourable to the idea, according to a senior Lib Dem source.

Danny Alexander, the chief secretary to the Treasury, told the BBC1 Sunday Politics show that the scheme was focused on "intergenerational fairness" to help those who could not currently help their children with deposits in the way his parents had done with him.

"There are an awful lot of parents who don't have enough cash to help their children get on the housing ladder. In many cases, they might well have built up a substantial pension pot – that's their only asset – which they will be able, when they reach retirement age, to release a tax-free lump sum."